Process of developing electrostatic



Aug. ll, 1959 R. E. HAYFORD PRocEss 0E DEVELOPING ELEcIRosIATIc IMAGE WITH cHARcoAE Filed Jan. 25, 1955 INVENTOR Rif/l ara/.EJ/C

2m/Z JLM ATTORNEY hired States Patent hice 2,899,331 Patented Aug. 11, 1959 PROCESS OF DEVELOPING ELECTROSTATIC IMAGE WITH CHARCOAL Richard E. Hayford, Pittsford, N.Y., assigner to Halod Xerox Inc., a corporation of New York Application January 25, 1955, Serial No. 484,000

3 Claims. (Cl. 117-175) This invention relates to photography, and particularly to electrophotography sometimes referred to as xerography.

As is now well known, this method of photography and reproduction usually comprises the use of a photoconductive material which, on being placed in an electric eld and exposed to radiation having a desired image or pattern, produces on a surface a latent image which is electrically charged, the charge being distributed in the same pattern as the original image. In xerography or electrophotography the latent image is then developed and toned by contacting the surface with an electrically charged powder or dust, which adheres to various portions of the charged surface in different amounts depending on the intensity distribution of electrical charges on the surface.

Such photographic processes are described for example in United States Patents Nos. 2,221,776; 2,297,691; 2,357,809; 2,618,551 and others.

An object of this invention is to provide an improved dust or powder for use in developing and toning the latent or electronic image.

Other objects and features of the invention are the provision of novel procedure for the preparation of the improved dust or powder.

Additional objects and features of the invention are to provide novel xerographic development procedure utilizing the improved dust or powder.

Other objects of my invention will be made clear by the following description taken in connection with the appended claims and the accompanying drawing which illustrates diagrammatically one mode of practicing the invention with the improved dust.

The preferred powders used in electrophotography are those having triboelectric properties, that is to say, powders or particles which have the property of acquiring an electric charge on frictional contact with selected materials. One such powder is finely ground wood charcoal, which acquires a net negative charge in triboelectric relations to many metals, for example by blowing the powdered charcoal through a metallic orifice. When such a powder is blown against a plate or surface containing an electrical charge pattern or latent image, it will adhere to positively charged portions of the plate, and the quantity of powder remaining in contact with the plate will depend on the intensity of the charge. However, such charcoal as hitherto used in even the highest grades available has certain disadvantages, including a tendency to agglomerate which gives coarse grained images. Moreover, it is frequently required, because of the nature of the charges on the photoelectric image, to use a positively charged instead of a negatively charged particle.

In distinction to wood charcoal, commercial carbon blacks tend to have more positive particles, but they agglomerate more severely than wood charcoal, resulting in even greater diiiculties due to agglomeration. They also give poor tonal density of the finished product.

The present invention in one aspect provides a process for preparing a novel charcoal which can be more readily positively rather than only negatively charged by triboelectric contact with certain metals and which has a lesser tendency to agglomerate than the wood charcoal heretofore obtainable for purposes of electrophotography or xerography. In addition, the new material is of particular value in xerography by virtue of an improved color tone, being more nearly a true black, whereas ordinary carbon materials, including ordinary charcoal, tends to have a sepia tone.

My improved wood charcoal can be used, for example, either in the so-called powder-cloud development process, in which it is suspended as a cloud in air or other gas, being charged, for example, by frictional charging in passing in turbulent flow through a line passage, and being deposited on relatively more highly negatively charged portions of the exposed surface, or by the cascade process in which parts of the exposed surface have a positive charge and the novel charcoal of this invention is repelled by the positively charged portions and adheres to the neutral portions of the exposed surface.

In carrying out the process of this invention wood charcoal, of a particle size adapted to xerography, i.e. from about 0.1 to 20 microns along the largest dimension, and having a tendency to acquire a negative charge in triboelectric relation to metals, is heated, preferably in an electric furnace, under controlled conditions of temperature and a protective atmosphere as described below.

In the first stage of the heating, during which the temperature is increased from substantially room temperature to approximately 400 F., the wood charcoal is agitated by means of rollers or otherwise for the purpose of eliminating any air bubbles which may be present. At the same time, the charcoal is maintained in an atmosphere of nitrogen or other non-oxidizing or inert gas, which is constantly being supplied to the furnace or kiln in which the heating is taking place. The preferred rate of heating is such that approximately 11/2 hours is required to raise the temperature of the charcoal to the maximum temperature of treatment, which is preferably approximately 1000" F. During the heating of the charcoal volatile material is given off, and this can be detected in the nitrogen leaving the kiln or furnace by odori or by known analytical means. Such volatiles are particularly emitted at approximately 500 F. and again at approximately 750 F. during the heating of the wood charcoal. At the temperatures where the volatile material is given off, which will, of course, differ somewhat with the previous history of the charcoal undergoing treatment, it is preferred to maintain the temperature without further rise until the evolution of Volatile matter ends, at which time the temperature is again slowly raised.

After a temperature of approximately 1000 F. is reached, the wood charcoal is cooled slowly to room temperature, while still protected from oxidation by an atmosphere of nitrogen or other inert gas.

The wood charcoal after being treated in the above Amanner will be found to have had its triboelectric property changed, that is it will acquire a more positive or even net positive rather than a negative charge when frictionally contacted with metals. Thus, when wood charcoal so treated is frictionally contacted with brass or platinum, it will acquire a net positive charge.

The wood charcoal ytreated by the process of this invention also has a lesser tendency to agglomerate, and produces, on Xerographic developing and toning, blacker blacks than the wood charcoals hitherto known.

When referring to the triboelectric charge taken on by wood charcoal the use of the terms positive and negative are not intended to mean that all of the charcoal is so charged on triboelectric treatment, but rather that that is the net or predominant charge observable in such charcoal.

As a further embodiment of the invention, the new charcoal material may be washed in a solvent such as acetone, chloroform or other organic solvent, or the like. This may be accomplished, for example, by mixing the heat treated powder with the desired solvent and filtering and drying the powder, optionally in a non-oxidizing atmosphere. When such solvent-washed powder is employed in xerographic development, it has been found to be more easily cleaned from the xerographic plate surface.

In practicing xerographic development with the new material herein described, the development powder, e.g. powdered wood charcoal prepared by the process as herein described so as to be triboelectrically positive in relation to metals, and being either of the type that has not been washed or the type that has been washed with the solvents hereinbefore mentioned, is applied as a powder cloud to a latent image bearing surface of a xerographic element 11. This powder cloud is created, for example, in conventional ways as by blowing the treated charcoal powder material into a cloud in any of several known types of powder cloud generators 12. The resulting cloud is then usually charged and deagglomerated in known way as by passing it through a capillary tube 13 under conditions of turbulent flow. Optionally, further charging can be effected by passing the cloud through a corona discharge zone provided by the corona discharge elements 14. If desired, charging of the powder cloud may be effected solely by passage of the cloud from the cloud generator through a corona discharge zone without any prior passage through a capillary tube 13. It is preferable, however, to effect passage through the capillary tube 13 because of the deagglomeration such passage provides.

The charged powder cloud of treated charcoal is then introduced between the latent image bearing surface 10 of the exposed xerographic element 11 and a closely adjacent conductive development electrode 15 usually spaced about /GO inch from the image bearing surface 10. The space 16 between the two is shown on a greatly exaggerated scale in the drawing. The charged powder cloud particles of charcoal of this invention which are introduced into the space 16 deposit themselves on the image bearing surface 10 in accord with the electrostatic pattern of the latent image in said surface.

The conductive development electrode 15 may be biased by connection through a contact spring 17 and a variable resistor 18 to a terminal of a D.C. source such as battery B. The other terminal of battery B is connected through a switch 19 to the usually conductive backing member 20 of the xerographic element 10. When the switch 19 is closed, resistor 18 can be adjusted to bias electrode 15 as desired. If the development electrode 15 is biased at substantially the potential of the highest charge on the image surface and if the powder particles are charged to the same polarity as the latent or electric image on the surface 10 of element 11, a condition known as reversal development of the latent image occurs. Such reversal development is distinguished from normal xero graphic development in that light objects become recorded as black and black objects become recorded as light. This is, of course, directly reverse to normal xerographic development. In effect reversal development is akin to photographic negative development. Because of the confusion likely to occur in the use of the term negative for this type of Xerographic development with the use of negative and positive to designate polarities of charges, the term reversal development is preferable. Its significance is as herein outlined. Because the treated wood charcoal produces blacker blacks than wood charcoals hitherto known, it is very effective in reversal development procedure in recording of light objects as blacker blacks.

The treated wood charcoal of this invention is of great utility likewise in normal xerographic development.

While specific embodiments of the invention have been described, variations within the scope of the appended claims are possible and are contemplated. There is no intention, therefore, of limitation to the exact details described and shown herein.

What is claimed is:

l. In a process for making visible an electrostatic image comprising areas of varying electrostatic charge on an electrically insulating surface by contacting said image with an aerosol of positively electrostatically charged finely-divided charcoal whereby said charcoal deposits on said insulating surface in conformity with said image, the improvement comprising generating said positively charged aerosol by passing through a brass orifice in turbulent flow a gas stream containing therein finely-divided wood charcoal characterized by having been heated to about l000 F. in a non-oxidizing atmosphere.

2. In a process for making visible an electrostatic image comprising areas of varying electrostatic charge on an electrically insulating surface by contacting said image with an aerosol of positively electrostatically charged finely-divided charcoal whereby said charcoal deposits on said insulating surface in conformity with said image, the improvement comprising generating said positively charged aerosol by passing through a brass orifice in turbulent ow a gas stream containing therein nely-divided wood charcoal with a particle size between about 0.1 and 20 microns and characterized by having been heated to about 1000 F. in a non-oxidizing atmosphere.

3. In a process of making visible an electrostatic image comprising electrostatic charges of positive polarity variously distributed on an insulating surface to provide potentials which differ for different areas of said surface, closely spacing a conductive electrode to said insulating surface so as to draw the lines of force of said electrostatic image externally above Said surface and while maintaining said conductive electrode at a potential corresponding to the highest potential on the said insulating surface and of the same polarity as the potential on the said surface, introducing a gas suspension of positively charged powder particles between said conductor and said surface, the improvement comprising generating said positively charged particles by passing through a brass orifice in turbulent flow a gas stream containing therein finely-divided wood charcoal characterized by having been heated to about l000 F. in a non-oxidizing atmosphere.

References Cited inthe file of this patent UNITED STATES PATENTS 1,478,730 Brownlee et al. Dec. 25, 1923 1,945,907 Kroner et al. Feb. 6, 1934 1,957,314 Billings et al. May l, 1934 2,297,691 Carlson Oct. 6, 1942 2,357,809 Carlson Sept. 12, 1944 2,707,674 Sweitzer May 3, 1955 2,711,481 Phillips lune 21, 1955 2,725,304 Landrigan et al Nov. 29, 1955 2,784,109 Walkup Mar. 5, 1957 

1. IN A PROCESS FOR MAKING VISIBLE AN ELECTROSTATIC IMAGE COMPRISING AREAS OF VARYING ELECTROSTATIC CHARGE ON AN ELECTRICALLY INSULATING SURFACE BY CONTACTING SAID IMAGE, WITH AEROSOL OF POSITIVELY ELECTROSTATICALLY CHARGED FINELY-DIVIDED CHARCOAL WHEREBY SAID CHARCOAL DEPOSITS ON SAID INSULATING SURFACE IN CONFORMITY WITH SAID IMAGE, THE IMPROVEMENT COMPRISING GENERATING SAID POSITIVELY CHARGED AEROSOL BY PASSING THROUGH A BRASS ORIFICE IN TURBULENT FLOW A GAS STREAM CONTAINING FINELY-DI- 